FIG. 1 shows, in the form of blocks, an electromagnetic transponder read terminal and a transponder which communicates with this terminal.
On the read terminal side are generally found a series oscillating circuit 2 formed of an inductance L1 forming an antenna in series with a capacitor C1 coupled between an output terminal 3 of an amplifier or antenna coupler (not shown) and a reference terminal 4 (generally, the ground). Generally, a resistor (not shown) is interposed between the antenna coupler output and inductance L1. The antenna coupler belongs to one or several circuits 5 (LECT) for controlling the oscillating circuit and exploiting received data and comprises, among others, a modulator-demodulator and a microprocessor for processing the control and data signals. In the example of FIG. 1, a node 6 of interconnection between capacitor C1 and inductance L1 forms a terminal for sampling the data signal received from transponder 10, intended for the demodulator of terminal 1.
Circuit 5 of the terminal generally communicates with different input-output circuits (keyboard, screen, means of transmission to a central server, etc.) and/or processing circuits not shown. The circuits of read/write terminal 1 extract the power necessary to their operation from a supply circuit (not shown) coupled, for example, to the electric system or to a battery.
On the side of transponder 10, an inductance L2, in parallel with a capacitor C2, forms a parallel oscillating circuit (called a resonant circuit) intended to sense the electromagnetic field generated by the series oscillating circuit (L1, C1) of terminal 1. The resonant circuit (L2, C2) of transponder 10 is tuned to the resonance frequency of oscillating circuit L1C1 of terminal 1. Parallel resonant circuit L2C2 is intended to capture the electromagnetic field generated by series oscillating circuit L1C1 of terminal 1.
Terminals 11, 12 of resonant circuit L2C2, corresponding to the terminals of capacitor C2, are coupled to two A.C. input terminals of a rectifying circuit 13 (RED) formed, for example, of a bridge (not shown) of four diodes for a fullwave rectification. It should be noted that circuit 13 may be replaced with a halfwave rectifying assembly. A capacitor Ca is coupled to the rectified output terminals 14 and 15 of circuit 13 to store the power and smooth the rectified output voltage.
When transponder 10 enters the electromagnetic field of terminal 1, a high-frequency voltage is generated across resonant circuit L2C2. This voltage, rectified by circuit 13 and smoothed by capacitor Ca, becomes a supply voltage (VD) on terminal 14 intended for electronic circuits 20 of transponder 10. Voltage VD is generally regulated by a voltage regulator (not shown). Circuits 20 (CTL) generally comprise a microprocessor, a memory, and a control circuit. For simplification, reference will be made hereafter to a single circuit 20 containing all these elements. Transponder 10 is generally synchronized by means of a dock (CLK) extracted, by a block 18, from the high-frequency signal recovered across capacitor C2, before rectification. Circuit 20 contains a means for demodulating signals 17 (DMOD) coming from terminal 1 and sampled on terminal 14. To transmit data from transponder 10 to terminal 1, circuit 20 controls a stage of modulation (called back-modulation) of resonant circuit L2C2. This modulation stage is generally formed of an electronic switch S and of a resistor R, in series between terminals 14 and 15. Transistor S is controlled by pulses at a so-called sub-carrier frequency (for example 424 kHz), smaller than the frequency of the excitation signal of oscillating circuit L1C1 of terminal 1 (for example, 13.56 MHz). Circuit 20 provides an output signal 19 (RMOD) of pulse control of switch S. Circuit 20 and the back-modulation stage altogether form a variable load of the transponder, equivalent to a resistor added in parallel on capacitor Ca. This equivalent resistor will be designated hereafter as Req.
When switch S is on, the oscillating circuit of the transponder is submitted to an additional damping with respect to the load formed of circuit 20, so that the transponder samples a greater amount of power from the high-frequency electromagnetic field. Accordingly, the power variation of transponder 10 translates, on the side of terminal 1, as an amplitude and phase variation of the current in antenna L1. This variation is detected in the signal present on node 6 and intended for the demodulator of terminal 1. The oscillating circuits of terminal 1 and of transponder 10 are generally tuned to the carrier frequency, that is, their resonance frequency is tuned to the 13.56-MHz frequency. Generally, all the electronic circuits of transponder 10 are integrated in a same chip.
FIGS. 2A and 2B illustrate, in the form of timing diagrams, a conventional example of the operation of transponder 10 in a data transmission to terminal 1. FIG. 2A shows an example of the shape of control signal RMOD of switch S provided by circuit 20 at a sub-carrier frequency of period (T). In this example, the data are coded by trains of four pulses. Other shapes are possible. Generally, the conduction duty ratio of switch S is set to 0.5.
FIG. 2B illustrates the shape of rectified voltage VD present on terminal 14. Each turning-on of switch S forms an additional charge sampled from the power reserve stored in capacitor Ca. Back-modulation resistor R is selected to cause a power variation of transponder 10 such that the resulting current amplitude and phase variation in antenna L1 is exploitable by circuit 5 of terminal 1. Generally, the internal consumption current of transponder 10 increases as soon as switch S, by its turning-on, reduces equivalent resistance Req by a ratio of from three to six. This current consumption increase causes a decrease in voltage VD.
Conversely, each turning-off of switch S causes an increase in voltage VD until the next turning back on of switch S. In the example of FIG. 2B, the variation ratio of resistance Req and thus of the internal consumption current of the transponder has been selected to be close to three. Thus, voltage VD generally decreases three times faster during on periods of switch S than it increases during its off periods. This weakening of voltage VD is all the more significant as transponder 10 is loosely coupled with terminal 1, that is, when the antennas are distant from each other (by more than sixty centimeters). As a result, the regulator is no longer able to provide a sufficient supply voltage to circuit 10. In FIG. 2B, the minimum threshold of voltage VD short of which regulator “uncouples” and circuit 10 is no longer powered has been designated as VDMIN. In the example of FIG. 2B, it is assumed that voltage VD reaches minimum value VDMIN at the end of a train of four pulses.
A problem which is posed when the oscillating circuits of the transponder and of the terminal are distant is that the power sampled by the back-modulation circuit is such that the transponder supply voltage is decreased to the point that its internal circuits stop operating properly. This results, on the one hand, in stopping the data transmission from the transponder to the terminal and, on the other hand, in reducing the transponder remote-supply distance.
Another problem is that, to properly demodulate the received signal, a reader which is sufficiently sensitive to operate while voltage VD is close to VDMIN typically must be provided. This results in a risk of demodulator saturation when, conversely, voltage VD is higher, that is, when the transponder is very close to the reader.